Coherent light-matter interactions in ultracold media

Cold atoms, as provided in a magneto-optical trap (MOT), are appropriate media to drive quantum memories and quantum computations. Coherent-adiabatic interactions serve as powerful tools to drive and manipulate such quantum memories. Examples for such robust and efficient interactions are, e.g. stimulated Raman adiabatic passage (STIRAP) or electromagnetically-induced transparency (EIT). In a new branch of our research operation, we aim at the development, implementation and application of novel coherent adiabatic interactions in laser-cooled atoms at extreme optical depths (ODs). Here, the OD is given by OD=-ln(T), where T is the transmission through the medium. Specifically, we aim at creating strongly-correlated light-matter systems by inducing interactions at the level of individual photons. Such strong interactions can be induced using EIT-based stationary light pulses (SLPs), i.e., light pulses with vanishing group velocity, in media of extreme OD.

Technical developments

To generate extreme ODs in excess of 1000, we load ultracold rubidium atoms from a magneto-optical trap (MOT) into the micron-sized core of a hollow-core photonic crystal fiber (HCPCF) (see Fig. 1) [2]. Inside the core, the atoms are guided by an optical dipole trap which prevents collisions of the ultracold atoms with the room-temperature fiber wall. This method allowed us recently to demonstrate for the first time an OD of 1000 with ultracold atoms. At such huge OD the medium becomes so opaque that no transmission is observed over a large frequency range (see Fig. 2).

Fig. 2 Absorption spectrum on the transitions |F=1>-> |F=0, 1, 2> versus probe laser detuning from state |F=1> (symbols). The black line shows a spectrum for OD=(300,1000,1000) on the transitions |F=1> ->|F0=0, 1, 2>. The gray line shows a spectrum for OD=1 for reference.

Future experiments

In the future we plan to utilize the fiber-based medium of extreme OD to perform experiments based on EIT. EIT renders an otherwise opaque medium transparent (see Fig. 3) while simultaneously significantly reducing the group velocity of light pulses. Besides the typical light storage experiments we specifically will perform experiments on SLPs. The dense atomic medium, driven by counter-propagating laser fields, here acts like a resonator cavity to "trap" a light pulse - but without the need for mirrors. Theoretical proposals already discussed a multitude of fascinating experiments based on SLPs. Among these are the generation of strongly-correlated light-matter systems, which, e.g., allow to drive repulsive and attractive interactions between otherwise non-interacting photons. Applications of such strong light-matter interactions are, e.g., all-optical quantum gates in future quantum networks.